Page 5 - Characterization and calibration of the Thomson scattering diagnostic suite for the C-2W field-reversed configuration experiment

Nucl. Fusion 58 (2018) 126026 B.H. Deng et al
Figure 10. Signals from magnetic probes installed in the azimuthal angles from 0° to 315° in a plane 1.21 m to the north of the CV mid- plane. Signals have been 5 kHz high-pass filtered and diametrically opposed pairs have been vertically shifted and grouped together for clarity.
Figure 11. Signals from magnetic probes installed in the same azimuthal angle of 0° but at 0.24 and 1.21 m to the north of the mid-plane, respectively.
Plotted in figure 11 are micro-burst time traces from two magnetic probes installed at 0.24 m and 1.21 m to the north of the mid-plane in the same azimuthal angle of 0°. The ver- tical dashed lines indicate that the oscillations are in phase, suggesting that kz ~ 0, i.e. the micro-burst activity is uniform along the CV axis. This observation and the n = 2 mode structure shown above are consistent with the theory to be dis- cussed in section 3.2.
2.3. Correlation between beam energy and micro-bursts
The FRC micro-burst instability appears to be largely non- destructive to bulk plasma confinement and has a very minor effect on fast ion confinement. The density profile is more peaked after each micro-burst as shown in figure 12. This pro- file peaking can be explained as some small fractional fast ion loss during micro-bursts: as the external magnetic field pres- sure is constant, the bulk plasma is compressed to higher pres- sure to balance the loss of fast ion pressure. It also suggests
Figure 12. Equilibrium density profile peaking after each micro- burst. The vertical line indicates the excluded flux radius at time of the burst. A large amplitude burst event is selected to be able to clearly show the density profile change through the burst.
that the bulk plasma pressure gradient is not the driving force of micro-bursts. The drive is from fast ions injected into and confined in the C-2U plasmas, as in FRC discharges without NBI, or with NBI but under discharge conditions leading to poor fast ion confinement and fair bulk plasma confinement, micro-bursts are either not observed or reduced significantly in amplitude, such as in the case when the n = 1 wobble mode is not completely suppressed.
Although micro-bursts are driven by fast ions, the correla- tion with NBI power is negative. An experimental evidence is that when the NBI injection energy is lowered from 15keV to 12.5keV with the total NBI power lowered from 9 MW to 6 MW correspondingly, the micro-bursts have earlier onset time and the burst interval reduces, as shown in figure 13. This experiment suggests that the micro-burst depends on beam energy. In another set of experiments the beam energy is varied from 8 to 12keV under the same plasma operation conditions, and the time traces of excluded flux radius from three consecutive shots are plotted in figure 14. All three shots are high performance FRC shots with about 7 ms plasma life- time, indicating the effectiveness of NBI in suppressing the
5